1. Field of the Invention
The invention generally relates to electronics. In particular, the invention relates to communication systems in which analog filters are tuned.
2. Description of the Related Art
Analog filters are widely used in radio transceivers. Typically, a communication device uses assigned channels with given bandwidth. In a transmitter, analog filters are employed to reduce signal leakage to adjacent channels and meet the requirements of a transmission spectral mask. In a receiver, analog filters are used to pass signals in the desired channel while suppressing interference from other channels. Requirements for analog filters in radio transceivers can be specified by filter parameters such as time-constant, cutoff frequency, passband flatness, stopband attenuation, group delay, etc. Since an analog filter can vary significantly with manufacturing process and temperature variation, an analog filter should be tunable to accommodate the variability. In addition, wireless communication systems have been moving towards a single device that can support multiple standards and operate in various environments. This also requires analog filters to be tunable and configurable to ease system complexity and reduce cost.
In phase-response based filter tuning methods, an analog filter is tuned such that the phase shift between the test signal and the filtered signal matches a desired phase shift. See, for example, U.S. Pat. No. 7,103,334 to Kumar and A Digital Automatic Tuning Technique for High-Order Continuous-Time Filters, by Taner et al, IEEE Transactions on Circuits and Systems I, vol. 51, no. 10, pp. 1975-1984, October 2004. A test signal at a pre-defined limited number of frequencies is used to generate a filtered signal. This way, a filter is tuned at only the specified frequencies regardless of its amplitude response and phase response at other frequencies. However, it may not be practical to isolate an analog filter from the rest of circuits in a system. Circuits other than the filter being tuned in the signal path can cause a phase shift of the filtered signal. This can lead to an inaccurate estimation of the actual phase shift due to the analog filter, and, therefore to a degradation in tuning performance.
In time-constant-based filter tuning methods, an analog filter's cutoff frequency is tuned by measuring and adjusting a time constant associated with the cutoff frequency. See, for example, U.S. Pat. No. 7,057,451 to Lou, et al., A Mixed-Signal Approach for Tuning Continuous-Time Low-Pass Filters, IEEE Transactions on Circuits and Systems II: Express Briefs, vol. 51, no. 6, pp. 307-314, June, 2004, Anthony et al, and U.S. Pat. No. 7,002,404, by Gaggl, et al.
Since the time constant is determined by the product of resistance and capacitance values in the filter circuits, the filter tuning result is sensitive to other circuits in the signal path used for measuring the time constant. Moreover, the time-constant based methods are typically only suitable for the tuning of single-stage RC filters. In general, a multi-stage analog filter has several low-order analog filters, which are separated by other circuits in a system. Thus, both the phase-response and time-constant based methods can only tune a multi-stage analog filter stage by stage, which leaves the combined overall performance uncertain.
In master-slave tuning methods, an analog filter is tuned at the cost of using an additional analog master filter that is identical in design to the operating analog filter. See, for example, A Single-Chip Dual-Band Tri-Mode CMOS Transceiver for IEEE 802.11a/b/g Wireless LAN, by Masoud et al, IEEE Journal of Solid-State Circuits, vol. 39, no. 12, pp. 2239-2249, December, 2004 and U.S. Pat. No. 7,078,960 by Ezell. Instead of tuning the operating slave filter, the master analog filter is tuned, and the tuning results are then applied to the slave filter. The advantage of master-slave methods is that the slave filter in use can be tuned without interrupting ongoing communications. However, these methods require a relatively large die area, and variations between the master and slave can impair the accuracy of the actual tuning.